cheat sheets.

** Adding a submodule to a superproject.

git submodule add <repository>
  Add the given repository as a submodule to the current git project.

** After cloning the superproject, the submodule directories will exist, but
they will be empty.

git submodule status
  Show the status of the submodules.

** Actually pulling down the submodules is a two-step process.

git submodule init

git submodule update

** Gotcha: submodules do not checkout master by default, it checks out a
specific commit relative to the superproject (somewhat like a tag).  To do any
work, make sure you checkout master or another branch.

$ cd my_submodule
$ git branch
* (no branch)
  master
$ git checkout master

** Gotcha: when publishing (pushing) make sure you publish the submodule changes
first, followed by the superproject.  Otherwise others will have problems
cloning the repository.

$ git commit -m "Updated the my_submodule from within the superproject."
$ git push
$ cd ..
$ git add my_submodule
$ git commit -m "Updated my_submodule."
$ git push